design flaw: using a struct as an interface and @fieldParentPtr can lead to bad pointer dereference

Reported here: https://github.com/scurest/zimodre/commit/8df208d42421693492668befd7811ea2761aa95f#commitcomment-25444745

Any struct that uses `@fieldParentPtr` on an "interface" field relies on the API user not making a copy of the interface field.

